1. Field of the Invention
The present invention generally relates to a piezoelectric thin-film resonator, and more particularly, to a filter using the piezoelectric thin-film resonator.
2. Description of the Related Art
As wireless communication devices such as mobile-phone handsets have widely spread, there is an increasing demand for filters and duplexers with higher performance. Filters and duplexers using piezoelectric thin-film resonators have characteristics with low loss and high power resistance, and therefore, are drawing attention as next-generation filters and duplexers to replace surface acoustic wave filters. The reason that each piezoelectric thin-film resonator filter causes low loss and exhibits high power resistance lies in the simple structure of each resonator. With the resonators having simple structures, electrodes of reasonable size can be maintained at higher frequencies, and deterioration in performance due to an increase of electric resistance can be avoided. However, the characteristics of surface acoustic wave filters have dramatically improved recently. To compete with them, it is necessary to develop piezoelectric thin-film resonator filters with higher performance and lower loss. In this trend, development to produce piezoelectric thin-film resonators that cause lower loss is being actively promoted.
Piezoelectric thin-film resonators of this type are generally known as FBAR (film bulk acoustic resonators), and are disclosed in Japanese Unexamined Patent Publication Nos. 2004-120494 and 2002-372974, and U.S. Pat. No. 6,657,363, for example.
One of the reasons of the loss caused in a piezoelectric thin-film resonator filter is that surface acoustic waves leak to the outside of the region formed by the upper electrode and the lower electrode facing each other (the region being referred to as the “resonator portion”, the outside region being referred to as the “non-resonator portion”). Surface acoustic waves cannot be re-converted into electric signals at the non-resonator portion, and result in low. This is referred to as “horizontal leakage”. The cause of the horizontal leakage lies in the sound velocity ratio between the resonator portion and the non-resonator portion. Such a sound velocity ratio as to prevent the horizontal leakage is determined by the Poisson's ratio of the piezoelectric film employed in the filter. If the Poisson's ratio is higher than 1/3, the sound velocity of the resonator portion is lower than the sound velocity of the non-resonator portion. If the Poisson's ratio is lower than 1/3, the sound velocity of the resonator portion is higher than the sound velocity of the non-resonator portion. With a piezoelectric film having a Poisson's ratio of 1/3 or higher, the sound velocity of the resonator portion can be made lower than the sound velocity of the surrounding region by performing mass addition on the resonator portion. In this manner, the horizontal leakage can be prevented easily.
However, if the Poisson's ratio of the piezoelectric film is lower than 1/3, the relationship in the sound velocity is reversed, and the horizontal leakage cannot be easily prevented. In each piezoelectric thin-film filter that is currently used in practice, AlN with a Poisson's ratio lower than 1/3 is employed as the piezoelectric film. Because of this, it is difficult to prevent the horizontal leakage, and the loss becomes even larger.